Abstract
<jats:p>We present a model for an OCD patient's response to SSRI treatment that combines serotonin pharmacokinetics, a basal ganglia-thalamocortical circuit, and corticostriatal plasticity, read out as a Y-BOCS score. Within it, that response is dictated by a plasticity-dynamics equation in which the OCD state is an elevated fixed point, separated from the healthy state by a saddle. This healthy-saddle-OCD structure is what leaves patients stuck. Effective treatment annihilates the saddle and the OCD fixed point together, making full remission possible. A response can occur even when annihilation does not, but those patients relapse once treatment is withdrawn. Calibrated to SSRI trial trajectories, the model shows that SSRIs achieve annihilation in mild-to-moderate OCD but not the most severe. Antipsychotic and memantine augmentation enter the dynamics at points distinct from the SSRI, so the model predicts they combine to annihilate the attractor an SSRI alone cannot, a rationale for augmentation in severe OCD.</jats:p>